The IEEE (Institute of Electrical and Electronics Engineers) 802.16 standards propose using an Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. OFDMA has also been proposed for use in 3GPP (Third Generation Partnership Project) Evolution communication systems. In an OFDMA communication system, a frequency bandwidth is split into multiple frequency subcarriers that comprise the physical layer channels over which traffic and signaling channels are transmitted in a TDM or TDM/FDM fashion. A user may then be assigned one or more of the frequency subcarriers for an exchange of bearer information, thereby permitting multiple users to transmit simultaneously on the different set of subcarriers such that each user's transmission is orthogonal to the other users transmissions and thus intra-cell interference is minimized.
In order to maximize bandwidth usage, OFDMA communication systems often engage in frequency selective scheduling. That is, for any given radio frame, the subcarriers may be allocated to users based on measured channel conditions. Alternatively, individual subcarriers across the entire bandwidth may be assigned to users so that a user can report an average CQI across all of its assigned subcarriers. Further, an appropriate modulation scheme and coding scheme may be determined for each subcarrier and each radio frame based on the measured channel conditions. The channel condition measurements are performed by a subscriber station (SS), which SS measures channel conditions for each assigned subcarrier or alternatively averages all its assigned subcarriers during a measuring period, such as a radio frame transmission period, and then reports the measured channel conditions for all of the subcarriers to a serving Node B in a Channel Quality Information (CQI) message. The CQI messages are conveyed at a fixed periodicity, or rate, typically every 8 frames, that is, every 40 milliseconds (ms).
Based on the reported CQIs, an OFDMA communication system selectively schedules the subcarriers each radio frame and further adaptively determines appropriate modulation and coding schemes for each subcarrier during the scheduling period. Reporting a CQI at a fixed periodicity may consume a significant amount of uplink system overhead, especially when there are a large number of reporting subscriber stations (SSs). In order to conserve bandwidth, it has been proposed that an SS report a CQI only when the CQI is above a threshold or is below another threshold, in effect, turning the CQI feedback channel on and off. However, such a proposal does not provide a base station scheduler with the CQI precision required to make optimal scheduling choices.
Therefore, a need exists for a method and apparatus that provides channel quality information that provides a balance between reducing CQI overhead and providing the CQI precision and reliability required to make optimal scheduling choices.